Heretofore, various type fuel injectors and fuel injection systems have been known in the prior art which are applicable to internal combustion engines. Of the many types of fuel injection systems, the present invention is directed to unit fuel injectors, wherein a unit fuel injector is associated with each cylinder of an internal combustion engine and each unit injector includes its own drive train to inject fuel into each cylinder on a cyclic basis. Normally, the drive train of each unit injector is driven from a rotary mounted camshaft operatively driven from the engine crankshaft for synchronously controlling each unit injector independently and in accordance with the engine firing order.
Of the known unit injectors of such fuel injection systems, there are two basic types of unit injectors which are characterized according to how the fuel is metered and injected. A first type to which the present invention is oriented is known as an "open nozzle" fuel injector because fuel is metered to a metering chamber within the unit injector where the metering chamber is open to the engine cylinder by way of injection orifices during fuel metering.
In contrast to the open nozzle type fuel injector, there are also unit fuel injectors classified as "closed nozzle" fuel injectors, wherein fuel is metered to a metering chamber within the unit injector while the metering chamber is closed to the cylinder of an internal combustion engine by a valve mechanism that is opened only during injection by the increasing fuel pressure acting thereon. Typically, the valve mechanism is a needle type valve.
In either case, the unit injector typically includes a plunger element that strikes the metered quantity of fuel to increase the pressure of the metered fuel and force the metered fuel into the cylinder of the internal combustion engine. In the case of a closed nozzle injector, a tip valve mechanism is provided for closing the injection orifices during metering with the tip valve biased toward its closed position thereby insuring that injection will take place only after the fuel pressure is increased sufficiently to open the tip valve mechanism.
The present invention is directed to the open nozzle type fuel injector, and more specifically to a unit injector fuel injection system that relies on pressure and time principles for determining the quantity of fuel metered for each subsequent injection of each injector cycle. Moreover, the pressure time principles allow the metered quantity to be varied for each cyclic operation of the injector as determined by the pressure of the fuel supplied to the metering chamber and the time duration that such metering takes place.
Such open nozzle fuel injectors are constantly subject to modifications in order to comply with increasingly higher levels of pollution control and the need for greater fuel economy. From this standpoint, such open nozzle unit fuel injectors have been developed, designed to comply with these requirements while at the same time providing a fuel injector of simplified design with associated cost reductions, and providing reliable and precise control of independently variable fuel injection timing and quantity parameters.
Examples of unit injectors of the open nozzle type are described in detail in U.S. Pat. Nos. 4,280,659 and 4,601,086 to Gaal et al. and Gerlach, respectively, both of which are owned by the assignee of the present invention. The injectors of Gaal et al. and Gerlach include a plunger assembly with a lower portion having a major diameter section that is slidable within an axial bore of the injector body and a smaller minor diameter section that extends within a cup of the injector body. The cup provides an extension to the axial bore which is smaller in diameter than the diameter of the axial bore that passes through the remainder of the injector body. During the metering stage of the Gaal et al. and Gerlach injectors, fuel is metered through a supply port into the axial bore at a point above the cup, and the fuel flows around the minor diameter section of the plunger assembly at the tip thereof for metering a specified quantity of fuel into the metering chamber of the cup. A radial gap is formed between the minor diameter section of the plunger assembly and the inner wall of the bore within the cup. This gap facilitates the flow of fuel to the injector tip to be injected. Once the metering stage is completed, the plunger travels inwardly (defined as toward the engine cylinder of an internal combustion engine) so as to cause injection of the fuel from the metering chamber through the injection orifices.
The stage just after the fuel injection has been completed is known as the crush stage, wherein the plunger tip is held tightly against a seat of the cup by the associated drive train for the unit fuel injector. During this crush stage, fuel is trapped within the radial gap between the minor diameter section of the plunger and the inner wall of the bore within the cup, as well as between the major diameter section of the plunger assembly and the end of the axial bore of the injector body. This quantity of fuel is known as the trapped volume.
A problem that is unique to such open nozzle type unit fuel injectors is that many of these known injectors permit a "secondary injection", which is leakage of fuel from the injector after injection should have been stopped. Such secondary injection is due to the trapped volume of fuel that is under high pressure near the bottom of the plunger tip. Such high pressure has the effect of forcing the plunger outwardly (that is defined as "away from" the engine cylinder) after the plunger is fully advanced to close the injection orifices by the cam. Thus, outward movement of the plunger and unseating of its tip allow trapped fuel leakage into the cylinder. Consequently, there is an amount of unburned and partially burned fuel remaining in the cylinder at the start of the exhaust stroke of a typical four-stroke cycle, which is due both to the abovementioned secondary injection as well as to inefficient combustion. This fuel specifically accounts for a large part of the visible smoke and unburned hydrocarbons emitted by the engine. It is, therefore, quite clear that the amount of harmful emissions would be reduced if the fuel injection were abruptly terminated at the point when combustion does not fully take place.
In order to more effectively seal the injection orifices from the metering chamber after injection, and to lessen the possibility of secondary injection, the injector plunger is typically driven by an amount further than when the plunger tip first contacts the seat of the injector cup. This travel is referred to as over-travel of the injector plunger which more tightly seals the plunger tip against the cup seat. However, the over-travel also raises the pressure of the trapped volume of fuel and still causes the aforedescribed tendency of the plunger tip to be lifted resulting in secondary injection. In fact, over-travel raises the pressure to at least partially counteract the effect of the over-travel. Another major disadvantage associated with over-travel is that the over-travel causes greater stress on the cam, plunger, and connection between the injector cup and barrel assembly. These stresses tend to cause injector failures due to excessive wear or breakage.
An attempt was made as described in U.S. Pat. No. 3,831,846 to Perr et al., which is owned by the assignee of the present invention, to eliminate secondary injection in an injector operating on the basis of plunger over-travel. In each of the disclosed embodiments, the plunger is divided into a plunger portion and a tip valve portion which are axially relatively movable by a small degree. The Perr et al. injector requires injector over-travel as a means to effectively close the injection orifices from the metering chamber after injection, particularly during the engine compression stroke, as in the above-noted prior art injectors. However, then to assist in holding the tip valve against the injection orifices and to prevent secondary injection, the trapped volume of fuel is utilized in addition to the hold down force from over-travel to hold the tip valve against the cup seat instead of forcing the tip away from the valve seat. This is accomplished by the axial separation between the main plunger assembly and the tip valve, wherein a pressure regulating passage through the tip valve, permits trapped volume near the injector tip to be forced upwardly near the upper end of the tip valve to act thereon and force the tip valve inwardly against the valve seat. Such movement is facilitated by the separation of the tip valve from the main plunger assembly and the pressure relief passage from the tip valve. Of course, the pressure of the trapped volume of fuel within the tip of the metering chamber is increased by the over-travel imparted to the tip valve by the plunger assembly from its associated drive mechanism. This over-travel purposefully being used to force trapped fuel through the pressure relief passage to act against the upper end of the tip valve.
Other similar injectors are also the subject of U.S. Pat. Nos. 4,249,499 to Perr and 4,149,506 to Muntean et al., both also commonly owned by the assignee of the present invention. These devices are based on the same tip principles as that described above with respect to the Perr et al. '846 device, which operate in unit injectors with plunger over-travel.
In a distinct other area of open nozzle injector modification for improving fuel injection for more efficient fuel burning and cleaner emissions, it has been suggested to increase the pressure of injection immediately at the injector tip. Such pressure increase has been found to more efficiently burn the injected fuel. One such example of a high pressure unit fuel injector is disclosed in U.S. Pat. No. 4,721,247 to Perr, also commonly owned by the assignee of the present invention. In this case, high SAC pressures in excess of 30,000 psi during injection have been encountered. The high pressure unit injector was designed to accommodate such high injection pressures without suscepting the injector to failure or destruction caused by the stresses associated with such high pressures. In order to do this, the injector was designed without a cup, per se, but combines the injector cup with a portion of the injector body which connects to the main injector body at a point above the high pressure zone of the metering chamber. Moreover, the Perr '247 injector includes a precise manner of controlling a timing chamber formed between separable portions of the plunger assembly for controlling the high pressure injection. By appropriately expanding and retracting the timing chamber, the start and finish of injection can be accurately controlled. Moreover, engine over-travel is absorbed by the collapse of the timing chamber as regulated to control the degree of flow of fuel from the timing chamber.
Yet another high pressure unit fuel injector is illustrated and described in co-pending application Ser. No. 402,893 filed Sep. 5, 1989 and also commonly owned by the assignee of the present invention. In this high pressure unit fuel injector, the timing chamber pressure is controlled by way of a low speed valve mechanism provided with a dual spring feature. The object of that invention is to more precisely control injection timing by way of the dual spring, low speed valve mechanism. Otherwise, the high pressure injector operates on the same principles enumerated above with respect to the Perr '247 high pressure unit fuel injector, in that plunger over-travel is controlled by the collapse of the timing chamber under the influence of the valve mechanism and various passages.
These high pressure open nozzle unit fuel injectors also suffer from the above-described problem of secondary injection even though they do not experience the pressure increases associated with over-travel because of the high pressures associated with the high pressure unit injectors themselves. In other words, the high pressure of the fuel trapped just after injection is sufficient in such a high pressure unit injector to cause secondary injection without overtravel of the plunger tip. Clearly, there is a need for a means to prevent secondary injection in such a high pressure open nozzle unit fuel injector.